156 Period. Polytech. Chem. Eng. D. Porowska

A Simple Method for Delineation of Leachate Plumes

Dorota Porowska1*

Received 26 June 2016; accepted after revision 10 October 2016

AbstractMeasurement and calculation of the carbon isotopic com-position of groundwater from piezometers located around a reclaimed landfill were performed in order to test the method to distinguish the piezometers localized within the contaminated area and to identify the boundaries of the leachate plume. In order to select the best method for delineation of the leachate plume it was analyzed: chemical composition, the stable car-bon isotopic composition in groundwater, and the calculation of carbon isotopic composition in groundwater. Comparison of the different methods for delineation of the leachate plume indicate, that the best method appear to be method based on the measurement and calculation of the carbon isotopic com-position in groundwater. The proposed method has been tested using the data from Otwock landfill (Poland), but it can also be used in other contaminated areas.

Keywordslandfill, groundwater, contaminated area, measured carbon isotope composition, calculated carbon isotope composition

1 IntroductionLandfill leachates contain a wide range of contaminants and

when they mix with natural groundwater, significant changes in groundwater quality are observed [1-6]. In most cases, leachate plumes are relatively small, less than a thousand meters long [7]. However, the delineation of a leachate plume around a munici-pal landfill may be sometimes complicated and may present sig-nificant challenges. Leachate composition varies significantly among landfills, depending on waste composition, volume, age and landfilling technology. The intensity of groundwater con-tamination by a leachate depends on hydrogeological condi-tions, climate and the engineering of the landfill [8]. Landfill leachate can create groundwater contaminant plumes that may last for decades to centuries [9, 10]. The load of contamination and its concentration in the groundwater are often spatially and temporarily variable, thus making identification of the bounda-ries of groundwater contamination zone complicated. Due to complexity of hydrogeological conditions and biochemical processes in a landfill, the assessment of the leachate plume extent requires detailed and unconventional measurements. The observation of the biogas emissions and lateral gas transport in soil adjacent to a landfill is a supplementary tool to demonstrate landfill impact on the environment [11-13]. Usually, the assess-ment of a leachate-contaminated area is based on the physical and chemical composition of groundwater, biochemical oxygen demand, chemical oxygen demand, total organic carbon, and heavy metals [2, 14-18]. Becker [19] estimated the vertical and horizontal extent of a leachate plume by performing a series of specific conductance measurements in monitoring wells located upgradient and downgradient of the landfill. Standard methods like chemical analysis of groundwater, hydrological descrip-tion, and geophysical studies (vertical electrical sounding and ground penetrating radar) were used for the identification and delineation of a contaminant plume in the shallow aquifer in Guadalupe Victoria landfill in Mexico [20]. Xenobiotic organic compounds (aromatic hydrocarbons, halogenated hydrocar-bons and phenols), originating from household or industrial chemicals are also good indicators of a landfill contamination [7, 21]. Observation of microbial communities in polluted

1 Faculty of Geology, University of Warsaw, Institute of Hydrogeology and Engineering Geology, Żwirki i Wigury 93, 02-089 Warsaw, Poland* Corresponding author, e-mail: dorotap@uw.edu.pl

61(3), pp. 156-162, 2017https://doi.org/10.3311/PPch.9667

Creative Commons Attribution b

research article

PPPeriodica PolytechnicaChemical Engineering

157A Simple Method for Delineation of Leachate Plumes 2017 61 3

and natural groundwater revealed that a long-term monitor-ing of the changes in their structure, together with monitor-ing of the changes in environmental conditions and alterations in groundwater hydrochemistry, might be a useful set of tools for identifying a contaminated area [22, 23]. Moreover, these observations facilitate the understanding of natural attenuation processes in the aquifer around the landfill.

Another method for detecting a contaminated area involves a delineation of the leachate plume based on stable carbon isotope composition in groundwater. The usefulness of carbon isotope determination in demonstrating landfill environmental impact was reported by North et al. [24]. Findings were supported by Haarstad and Mæhlum [25], who concluded that δ13CDIC is a more sensitive tracer than the content of organic matter, salts, nutrients and heavy metals in the leachate. Roslanzairi Mostapa et al. [26] compared the conventional and stable isotope tech-niques and found out that the isotope technique was more accu-rate in determining the distribution of landfill leachate in the groundwater and surface water, but it should be coupled with the conventional technique.

Sometimes it is difficult to delineate the leachate plume using conventional methods, as physicochemical composition of the groundwater. In the groundwater around the Otwock landfill blurs a clear division into two zones, especially in the case of sulfates [27]. The leachate-contaminated area may be more effectively identified by measurement of stable carbon isotope composition. When the leachates are only to small extent mixed with groundwater, the measured values alone are not enough to conclude whether the isotopic composition of groundwater results from the landfill impact or from natural changes in the environment, and additional interpretation is necessary. The aim of this study is to present a new method to apply and evaluate an unconventional method to select pie-zometers with leachate-contaminated groundwater and deline-ate the leachate plume. This method is based on a comparison of measured and calculated values of δ13CDIC in groundwater and leachate. Carbon isotope composition may be calculated using δ13CDIC mass balance, assuming an appropriate isotopic fractionation, and taking into account physicochemical compo-sition of groundwater [28-30]. The proposed method has been tested using the data from Otwock landfill, however is suit-able for wider application. The method can also be successfully used in other contaminated areas.

2 Materials and methods2.1 Site description

The study was conducted around a reclaimed municipal landfill, located in the suburbs of Otwock, about 25 km south-west of Warsaw (Poland) (Fig. 1). The landfill covers an area of 2.8 ha. It received municipal waste from the community of Otwock from 1961 to 1991. After closing, the landfill was cov-ered with a layer of soil, consisting of approximately 30-50 cm

of compost and planted with grass. The contamination of groundwater by leachate is from the infiltration and passage of water through the solid waste into the aquifer, because the landfill has no bottom liner.

Fig. 1 Location of the study area (A), arrangement of the piezometers and groundwater table contour (B)

The geological profile includes highly permeable river deposits composed of gravels and varigrained sands, with occa-sional organic matter or peat layers distributed within the aqui-fer. The hydraulic conductivity of the aquifer ranged between 6.5•10-4 m/s and 1.6•10-3 m/s, depending on the clay content. The general groundwater flow direction is northwest, towards the Vistula and Świder (Fig. 1). During the study period, the groundwater table in the measured piezometers ranged from ~1.5 m below ground (bg) to ~2.7 m bg (from spring to autumn), and from ~1.0 m bg to ~1.7 m bg (in winter).

2.2 Sampling and analytical proceduresField and laboratory research were conducted quarterly

from May 2006 to March, using the same research equipment and analytical procedures. Groundwater samples for chemical analyses were collected from piezometers around the landfill (Fig. 1). Water temperature and pH were measured in the field using the pH-meter 330i (WTW). Bicarbonate (HCO3

-) was measured on site by HCl titrating. Laboratory analyses included determination of the cations Mg2+, Ca2+, K+, and Na+ (in fil-tered and acidified samples) by Perkin Elmer atomic absorption spectrometry, and the anions SO4

2-, Cl-, NO3- were determined

by high-performance liquid chromatography (HPLC). The analyses were performed at the Central Chemical Laboratory of the Polish Geological Institute - National Research Institute

158 Period. Polytech. Chem. Eng. D. Porowska

in Warsaw and at the Laboratory of Institute of Hydrogeology and Engineering Geology, Faculty of Geology in Warsaw.

Unfiltered samples of groundwater from the aquifer were collected to determine δ13CDIC. The samples were conserved with SrCl2-NH4OH. The measurement of carbon isotopic com-position was determined using a Finnigan MAT Delta+ mass spectrometer in a Dual Inlet system at the Isotope Dating and Environment Research Laboratory of the Institute of Geological Sciences, Polish Academy of Sciences in Warsaw. δ13C values of the dissolved inorganic carbon samples were reproducible to ±0.1‰. The results were expressed with reference to the VPDB standard. Routine δ13CDIC measurements had an overall preci-sion exceeding 0.1‰.

Numerous groundwater components were analyzed for the Otwock landfill, but this paper presents only the parameters which are useful to the calculation of the carbon isotope com-position of groundwater.

3 Theoretical considerationsThe content of δ13C in organic matter ranges from -34 to

-24‰ [31], with average about -28‰ [32], and soil gas CO2(g)

average concentration is about -23‰ [28]. During conversion of soil gas CO2(g) into dissolved carbon dioxide CO2(aq), an iso-topic fractionation occurs and this enriches CO2(aq) with 13C by approximately -1.1‰, relative to the CO2(g). Therefore, δ13C of dissolved carbon dioxide CO2(aq) amounts to ca. -24.1‰. During conversion of dissolved carbon dioxide CO2(aq) into bicarbonate, the isotopic fractionation enriches HCO3

- with 13C by approximately 9‰, relative to CO2(aq). Therefore, δ13C of bicarbonate is -15.1‰. However, the values of δ13C in the leachates and leachate contaminated groundwater may also be positive, even up to +38‰ [33]. This simplified assessment of δ13C concentration is not adequate when other sources of carbon is considered. The assessment is more accurate when carbon isotope composition is calculated using δ13CDIC mass balance equation (Eq. (1)) [28-30].

δ δ ε13

2

13

2

13

2 2C mCO C C

mH

DIC calc aq CO g CO aq CO g− ( ) ( ) − ( )= +( )

+

. ( )

CCO C C

mCO C C

CO g HCO CO g

CO g CO

3

13

2

13

3 2

3

2 13

2

13

3

−( ) − ( )

−( )

+( )+ +

δ ε

δ ε −− ( ) ( )− −( ) + +( )CO g aqmCO mHCO mCO

2 2 3 3

2

where:mCO2(aq), mHCO3

-, mCO32- - molalities of the DIC species:

CO2(aq), HCO3-, and CO3

2-, respectivelyδ13CCO2(g) - carbon isotopic composition of soil gas CO2(g)

ε13CCO2(aq)-CO2(g), ε13CHCO3-CO2(g), ε13CCO3-CO2(g) - equilibrium fractionations.

This equation was used to calculate carbon isotopic compo-sition in groundwater. The values of equilibrium fractionation (ε) were adopted from Mook (ed.) [29] and Darling et al. [30],

and are listed in Table 1 (temperatures appropriate for the groundwater in Otwock). Equilibrium fractionation between the carbonate ion (CO3

2-) and carbon dioxide (CO2(g)) were also determined by Zhang et al. [34], Hałas et al. [35], Szaran [36], and Leśniak and Zawidzki [37]. Molalities of DIC species and other calculations were based on physicochemical composition of groundwater, i.e. water temperature, pH, and the concentra-tion of HCO3

-, SO42-, Cl-, Ca2+, Mg2+, Na+, K+ [28].

Table 1 The values of equilibrium fractionations (ε) used in the calculation

Water temperature

ε 13CCO2(aq)-CO2(g) ε 13CHCO3-CO2(g) ε 13CCO3-CO2(g)

1 -1.2 10.1 10.2

6 -1.1 9.6 9.6

7 -1.1 9.5 9.5

8 -1.1 9.4 9.4

9 -1.1 9.3 9.3

10 -1.1 9.2 9.2

11 -1.1 9.1 9.1

12 -1.1 9.0 8.9

13 -1.1 8.9 8.8

14 -1.1 8.8 8.7

4 Results and discussionIn order to select the best method for delineation of the lea-

chate plume was analyzed, i.e: the concentration of major ions, the stable carbon isotopic composition in groundwater, and the calculation of carbon isotopic composition in groundwater.

4.1 Delineation of the leachate plume using the chemical composition in groundwater

Separating between the zone of natural conditions and leachate-contaminated area resulting from an analysis of the major ions was not clear (Fig. 2). Unequivocal delineation of the contaminated area around the Otwock landfill is difficult due to gradual changes in the chemistry between the adjacent piezometers e.g. for bicarbonates or sulfates independent of the sampling site [27, 38]. Chemical composition of the ground-water changed gradually, and so the method of dividing the groundwater into zones depending on the concentration of the major ions is not appropriate in this case.

Similar concentration of sulfates, from 36.2 mg/dm3 to 106 mg/dm3, was found in the groundwater around the landfill (Fig. 3). According to the above-discussed division into two zones, sulfate concentration in the natural groundwater ranged from 36.2 mg/dm3 to 110 mg/dm3, and in the leachate-contami-nated water from 56 mg/dm3 to 106 mg/dm3. Therefore, sulfate concentration, as the concentration of other major ions, was unsuitable for assessing the extent of the leachate plume.

(1)

159A Simple Method for Delineation of Leachate Plumes 2017 61 3

Fig. 2 Chemical composition of groundwater around the Otwock landfill

Fig. 3 Distribution of sulfate concentration in groundwater around the Otwock landfill

A better method seems to be the comparison of calcium and bicarbonates concentrations in groundwater around the landfill (Fig. 4). This plot indicates which piezometers are not influ-enced by landfill contamination. The samples representing natural conditions are located along 1:1 line and suggest that chemical composition of groundwater (main ions: HCO3

-, Ca2+) is principally controlled by natural biogeochemical processes such as CO2 generation in the soil and aquifer through the decay of natural organic matter and calcite dissolution. Groundwater samples collected from contaminated area located below 1:1

line manifest high HCO3- concentration in comparison to Ca2+

concentration caused by anthropogenic processes (activity of the landfill). This division in not so clear in the case of con-taminated area, because sometimes the point may be located near the line 1:1, so this method is not appropriate in this case.

Fig. 4 Relationship between calcium and bicarbonates concentrations in groundwater around the Otwock landfill

4.2 Delineation of the leachate plume using the carbon isotopic composition in groundwater

The measured of carbon isotopic composition of groundwa-ter (δ13CDIC) varied widely, from -20.6‰ in piezometer No. 3 to +3.6‰ in piezometer No. 6 (Fig. 5). It could be expected that the groundwater downgradient from the landfill was contami-nated and the results from piezometers No. 2, 6, 22, 23, and 24 confirmed this assumption. Piezometers No. 1 and 3 were both localized upgradient from the landfill (Fig. 5), and thus repre-sents the natural groundwater. However, it is not clear whether the δ13CDIC of -12.4‰ detected in the piezometer No 1 in the win-ter indicates that the groundwater is contaminated by leachate. Similarly, it is not clear whether piezometer No. 9 belongs to the area of natural groundwater or the contaminated area, because δ13CDIC concentrations recorded there were -14.5‰ and -14.8‰, depending on the season. In general, natural groundwater is characterized by a wide range of δ13CDIC (from -30 to 0‰ but most commonly from -25 to -10‰) [28, 39], and a comparison of the results with the literature data does not provide answers to the question whether they represent the natural groundwater or the leachate-contaminated water.

Leachates are usually characterized by positive values of δ13CDIC (even up to +38‰) [33], but when they are only to small extent mixed with groundwater, it is difficult based on the measured values only, without additional interpretation to assess whether the isotopic composition of the groundwater is due to the landfill impact or to natural changes in the environ-ment, e.g. increasing share of carbonate dissolution naturally occurring in the aquifer.

160 Period. Polytech. Chem. Eng. D. Porowska

Fig. 5 Distribution of carbon isotope composition in groundwater around the Otwock landfill

4.3 Delineation of the leachate plume using the calculation of carbon isotopic composition in groundwater

δ13CDIC in the groundwater in Otwock was determined from the isotope mass balance equation (Eq. (1)), using equilibrium fractionation values (ε) from Table 1 and physicochemical composition parameters of the groundwater (water tempera-ture, pH, HCO3

-, SO42-, Cl-, Mg2+, Ca2+, Na+, K+). Measured

δ13CDIC varied widely, from -20.6‰ to +3.6‰, and the calcu-lated concentrations ranged from -19.3‰ to -13.7‰. Measured δ13CDIC were compared with respective calculated concentra-tions in Figure 6. Two groups, differing in the calculated and measured concentrations of δ13CDIC, could be identified in the landfill area. The first one represents carbon isotope composi-tion in the natural groundwater, and the second in the leachate-contaminated groundwater. In the natural groundwater, the points were arranged along 1:1 line, whereas in the leachate-contaminated groundwater they were placed below this line. This suggests that other sources of carbon were important in this type of groundwater and different processes have caused isotope fractionation.

The similar calculated and measured value of δ13CDIC in nat-ural conditions indicates that isotopic fractionation takes place in a generally known manner, and the values of equilibrium fractionations used in calculations and listed in Table 1 show the real fractionation occurring in this environment. In the lea-chate-contaminated area higher measured than calculated value of δ13CDIC indicated, that the landfill activity was a significant

source of carbon in this area, causing changes in carbon iso-tope fractionation. During anaerobic decomposition of organic matter in landfill occur high isotope fractionation. The values of equilibrium fractionations are different compared to natural conditions and individual in every landfill. This causes differ-ences in the calculated and measured values δ13CDIC as well as differences between the natural groundwater and the leachate-contaminated groundwater. Due to enrichment of 13C during methanogenesis, δ13CDIC of leachates may increase up to +10‰ [40] or even up to +38‰ [33], whereas in natural environment this value ranged from -25 to -10‰ [28].

Fig. 6 Relationship between δ13CDIC measured and δ13CDIC calculated

Mixing leachates and groundwater in the aquifer along the groundwater flow direction causes differentiation in carbon isotopic composition in the leachate plume. The concentration of δ13CDIC in the groundwater depends on the amount of the leachate and hydrogeochemical conditions within the aquifer. With this assumption in mind, it was possible to select the pie-zometers with natural groundwater and leachate-contaminated water. Piezometers No. 2, 6, 22, 23 and 24, located the closest to the landfill and along the groundwater flow direction shows contamination by the leachate (Fig. 7). Piezometers No. 1 and 3, located upgradient from the landfill belongs to the area of natural groundwater, as do piezometers No. 5 and 9, despite their location downgradient from the landfill.

The outcomes of δ13CDIC mass balance equation were consist-ent with earlier research using cross-plot of δ13CDIC versus 1/DIC.

According to the previous study [41], the research area is divided into two zones differing in the landfill activity. In the first, carbon is derived from the organic matter degradation and from carbonate dissolution in the natural groundwater. In the second, δ13CDIC concentration is controlled by the mixing ratio of organic matter degradation within the aquifer sediments and biodegradation of organic matter stored in the landfill.

161A Simple Method for Delineation of Leachate Plumes 2017 61 3

Fig. 7 Arrangement of the piezometers with natural groundwater and leachate-contaminated water

5 ConclusionsDelineation of the leachate plume using the calculation of

carbon isotopic composition in groundwater seems to be a suit-able and useful method. Knowing the chemical composition of groundwater, the equilibrium fractionation and the results of δ13CDIC mass balance, it is possible to calculate the carbon isotopic composition of groundwater. A comparison of the cal-culated and measured values of carbon isotopes δ13CDIC in the groundwater around the reclaimed landfill in Otwock unequiv-ocally demonstrated that it is an effective method of select-ing the piezometers with leachate-contaminated groundwater and delineating the leachate plume. The proposed method was tested in the research performed around the Otwock landfill, but it can be used in other areas, contaminated by leachate. According to this method, the piezometers No. 2, 6, 22, 23, and 24, located downgradient from the landfill, were impacted by the leachate. Water in the piezometers No. 1 and 3, located upgradient from the landfill, as well as the piezometers No. 5 and 9, despite their location downgradient from the landfill, could be classified as natural water.

The advantages of the proposed method compared with the other methods that are currently applied for delineating the area of contaminated groundwater are the following: 1) evi-dent selection of the piezometers with leachate-contaminated groundwater 2) no additional cost 3) no additional laboratory tests, 4) easy to realize 5) suitable for wider application in other area contaminated by leachate.

AcknowledgementThe project presented in this article is supported by the State

Committee for Scientific Research (project no. 4 T12B 061 28) and partly also by the Faculty of Geology, University of Warsaw.

References[1] Van Breukelen, B. M., Griffioen, J., Röling, W. F. M., Van Verseveld, H.

W. "Reactive transport modelling of biochemical processes and carbon isotope geochemistry inside in landfill leachate plume." Journal of Con-taminant Hydrology. 70, pp. 249-269. 2004.

https://doi.org/10.1016/j.jconhyd.2003.09.003[2] Arora, B., Mohanty, B. P., Mcguire, J. T, Cozzarelli, I. M. "Temporal dy-

namics of biogeochemical processes at the Norman Landfill site." Water Resources Research. 49, pp. 1–18. 2013.

https://doi.org/10.1002/wrcr.20484[3] Patil, S. B., Chore, H. S. "Contaminant transport through porous media:

An overview of experimental and numerical studies." Advances in Envi-ronmental Research. 3(1), pp. 45-69. 2014.

https://doi.org/10.12989/aer.2014.3.1.045[4] Mukherjee, S., Mukhopadhyay, S., Hashim, M. A., Sen Gupta, B. "Con-

temporary Environmental Issues of Landfill Leachate: Assessment and Remedies." Critical Reviews in Environmental Science and Technology, 45(5), pp. 472-590, 2015.

https://doi.org/10.1080/10643389.2013.876524[5] Masoner, J. R., Cozzarelli, I. M. "Spatial and temporal migration of a

landfill leachate plume in alluvium." Water Air and Soil Pollution. 226, article 18, 2015. https://doi.org/10.1007/s11270-014-2261-x

[6] Dervišević, I., Đokić, J., Elezović, N.,Milentijević, G., Ćosović, V., Dervišević, A. "The Impact of Leachate on the Quality of Surface and Groundwater and Proposal of Measures for Pollution Remediation." Journal of Environmental Protection. 7, pp. 745-759. 2016.

https://doi.org/10.4236/jep.2016.75067[7] Christensen, T. H., Kjeldsen, P., Bjerg, P. L., Jensen, D. L., Christensen,

J. B., Baun, A., Albrechtsen, H.-J., Heron, G. "Biogeochemistry of land-fill leachate plumes." Applied Geochemistry. 16(7-8), pp. 659-718. 2001.

https://doi.org/10.1016/S0883-2927(00)00082-2[8] Bjerg, P. L., Tuxen, N., Reitzel, L. A., Albrechtsen, H. J., Kjeldsen, P.

"Natural attenuation processes in landfill leachate plumes at three Danish sites." Ground Water. 49(5), pp. 688–705. 2011.

https://doi.org/10.1111/j.17456584.2009.00613.x[9] Cozzarelli, I. M., Böhlke, J. K., Masoner, J., Breit, G. N., Lorah, M.

M., Tuttle, M. L. W., Jaeschke, J. B. "Biogeochemical evolution of a landfill leachate plume, Norman, Oklahoma." Ground Water. 49(5), pp. 663–687. 2011. https://doi.org/10.1111/j.1745-6584.2010.00792.x

[10] Bjerg, P. L., Albrechtsen, H.-J., Kjeldsen, P., Christensen, T. H., Cozza-relli, I. M. "The Biogeochemistry of Contaminant Groundwater Plumes Arising from Waste Disposal Facilities." In: Treatise on Geochemistry. Holland, H. D., Turekian, K. K. (ed.), Elsevier, Oxford, pp. 573-605, 2014. https://doi.org/10.1016/B978-0-08-095975-7.00916-5

[11] Hettiaratchi, P., Jayasinghe, P., Tay, J. H., Yadav, S. "Recent advances of biomass waste to gas using landfill bioreactor technology - a review." Current Organic Chemistry. 19(5), pp. 413-422. 2015.

https://doi.org/10.2174/1385272819666150119223002[12] Christophersen, M., Kjeldsen, P., Holst, H., Chanton, J. "Lateral gas

transport in soil adjacent to an old landfill: factors governing emissions and methane oxidation." Waste Management & Research. 19(6), pp. 595-612. 2001. https://doi.org/10.1177/0734242X0101900616

[13] Townsend, T. G., Powell, J., Jain, P., Xu, Q., Tolaymat, T., Reinhart, D. "Sustainable Practices for Landfill Design and Operation." Springer New York, 2016. https://doi.org/10.1007/978-1-4939-2662-6

162 Period. Polytech. Chem. Eng. D. Porowska

[14] Kjeldsen, P., Barlaz, M. A., Rooker, A. P., Baun, A., Ledin, A., Christens-en, T. H. "Present and long term composition of MSW landfill leachate - a review." Critical Reviews in Environmental Science and Technology. 32(4), pp. 297-336. 2002. https://doi.org/10.1080/10643380290813462

[15] Van Breukelen, B. M., Röling, W. F. M., Groen, J., Griffioen, J., Van Ver-seveld, H. W. "Biogeochemistry and isotope geochemistry of a landfill leachate plume." Journal of Contaminant Hydrology. 65, pp. 245-268. 2003. https://doi.org/10.1016/S0169-7722(03)00003-2

[16] Zhang, Q. Q., Tian, B. H., Zhang, X., Ghulam, A., Fang, Ch. R., He, R. "Investigation on characteristics of leachate and concentrated leachate in three landfill leachate treatment plants." Waste Management. 33, pp. 2277-2286, 2013. https://doi.org/10.1016/j.wasman.2013.07.021

[17] Raco, B., Dotsika, E., Battaglini, R., Bulleri, E. Doveri, M., Papakostan-tinou, K. "A Quick and reliable method to detect and quantify contami-nation from MSW landfills: a case study." Water Air and Soil Pollution. 224, article 1380, 2013. https://doi.org/10.1007/s11270-012-1380-5

[18] Hossain, Md. L., Das, S. R., Hossain, M. K. "Impact of Landfill Leachate on Surface and Ground Water Quality." Journal of Environmental Sci-ence and Technology. 7, pp. 337-346. 2014.

https://doi.org/10.3923/jest.2014.337.346[19] Becker, C. J. "Hydrogeology and leachate plume delineation at a closed

municipal landfill, Norman, Oklahoma." University of Michigan Li-brary, pp. 1-52. 2002.

[20] Reyes-López, J. A., Ramírez-Hernández, J., Lázaro-Mancilla, O., Car-reón-Diazconti, C., Garrido, M. M. "Assessment of groundwater con-tamination by landfill leachate: a case in México." Waste Management. 28, pp. S33-S39. 2008. https://doi.org/10.1016/j.wasman.2008.03.024

[21] Baun, A., Reitzel, L. A., Ledin, A., Christensen, T. H., Bjerg, P. L. "Natu-ral attenuation of xenobiotic organic compounds in a landfill leachate plume (Vejen, Denmark)." Journal of Contaminant Hydrology. 65(3-4), pp. 269-291. 2003. https://doi.org/10.1016/S0169-7722(03)00004-4

[22] Milosevic, N., Thomsen, N. I., Juhler, R. K., Albrechtsen, H.-J., Bjerg, P. L. "Identification of discharge zones and quantification of contaminant mass discharges into a local stream from a landfill in a heterogeneous geologic setting." Journal of Hydrology. 446–447, pp. 13–23. 2012.

https://doi.org/10.1016/j.jhydrol.2012.04.012[23] Brad, T., Obergfell, C., van Breukelen, B. M., van Straalen, N. M.,

Röling, W. F. M. "Spatiotemporal variations in microbial communities in a landfill leachate plume." Groundwater Monitoring and Remediation. 33(4), pp. 69–78. 2013. https://doi.org/10.1111/gwmr.12022

[24] North, J. C., Frew, R. D., Van Hale, R. "Can stable isotopes be used to monitor landfill leachate impact on surface waters?" Journal of Geochem-ical Exploration. 88, pp. 49-53. 2006.

https://doi.org/10.1016/j.gexplo.2005.08.003[25] Haarstad, K., Mæhlum, T. "Tracing solid waste leachate in groundwater

using δ13C from dissolved inorganic carbon." Isotopes in Environmental and Health Studies. 49(1), pp. 48-61. 2013.

https://doi.org/10.1080/10256016.2012.668902[26] Mostapa, R., Syafalni, Abdul–Rahman, M.-T., Abustan, I. "Comparison

between conventional and stable isotope techniques in determining dis-tribution of landfill leachate in groundwater and surface waters in Perak, Malaysia – techniques of water resources investigation." International Journal of Environmental Studies. 1(5), pp. 955-965. 2011.

https://doi.org/10.6088/ijessi.00105020024

[27] Porowska, D. "Sulfur compounds in biogas and groundwater around reclaimed municipal landfill in Otwock." Polish Geological Review. 62(11), pp. 761-767. 2014. (in Polish)

[28] Clark, I. "Groundwater geochemistry and isotopes." CRC Press, Taylor & Francis Group, Boca Raton. 2015. https://doi.org/10.1201/b18347

[29] Mook, W. G. (ed.) "Environmental isotopes in the hydrological cycle. Principles and applications." Vol. I. Introduction, Theory, methods, re-view. Technical Documents in Hydrology No 39, Vol. I, UNESCO, Paris, pp.1-280. 2000.

[30] Darling, W., Bath, A. H., Gibson, J. J., Różański, K. In: Leng M. J. (ed.) "Isotopes in Palaeoenvironmental Research." Springer, Dordrecht, The Netherlands. 2005.

[31] Faure, G. "Principles of Isotope Geology." John Wiley & Sons, New York, pp. 589, 1986.

[32] Deines, P. "The isotopic composition of reduced organic carbon." In: Fritz, P., Fontes, J. C. (eds.). Handbook of Environmental Isotope Geo-chemistry. Elsevier, Amsterdam, pp. 329-406 (vol. 1). 1980.

[33] Grossman, E. L. "Stable carbon isotopes as indicators of microbial activ-ity in aquifers." In: Hurst C. J., Knudso, G. R., McInerney M. J., Stetzen-bach L. D., (eds.). Manual of Environmental Microbiology. ASM Press, Washington, DC, pp. 728-742. 1997.

[34] Zhang, J., Quay, P. D., Wilbur, D. O. "Carbon isotope fractionation dur-ing gas-water exchange and dissolution of CO2." Geochimica et Cosmo-chimica Acta. 59, pp.107-114. 1995.

https://doi.org/10.1016/0016-7037(95)91550-D[35] Hałas, S., Szaran, J., Niezgoda, H. "Experimental-determination of car-

bon-isotope equilibrium fractionation between dissolved carbonate and carbon-dioxide." Geochimica et Cosmochimica Acta. 61, pp. 2691-2695. 1997. https://doi.org/10.1016/S0016-7037(97)00107-5

[36] Szaran, J. "Carbon isotope fractionation between dissolved and gaseous carbon dioxide." Chemical Geology. 150, pp. 331-337. 1998.

https://doi.org/10.1016/S0009-2541(98)00114-4[37] Leśniak, P. M., Zawidzki, P. "Determination of carbon fractionation fac-

tor between aqueous carbonate and CO2(g) in two-direction isotope equi-libration." Chemical Geology. 231, pp. 203-213. 2006.

https://doi.org/10.1016/j.chemgeo.2006.01.025[38] Porowska, D. "Assessment of groundwater contamination around re-

claimed municipal landfill - Otwock area, Poland." Journal of Ecological Engineering. 15(4), pp. 69-81. 2014.

https://doi.org/10.12911/22998993.1125460[39] Porowski, A. "Isotope hydrogeology." In: Eslamian, S. (ed.), Handbook

of engineering hydrology. Fundamentals and applications. Taylor & Francis Group, CRC Press, Boca Raton London New York, pp. 345–377. 2014. https://doi.org/10.1201/b15625

[40] Wimmer, B., Hrad, M., Huber-Humer, M., Watzinger, A., Wyhlidal, S., Reichenauer, T. G. "Stable isotope signatures for characterising the bio-logical stability of landfilled municipal solid waste." Waste Management. 3(10), pp. 2083-2090. 2013.

https://doi.org/10.1016/j.wasman.2013.02.017[41] Porowska, D. "Determination of the origin of dissolved inorganic carbon

in groundwater around a reclaimed landfill in Otwock using stable car-bon isotopes." Waste Management. 39, pp. 216-225. 2015.

https://doi.org/10.1016/j.wasman.2015.01.044